The present invention relates to a photocatalyst for hydrogen production and preparation thereof and a method for producing hydrogen by use of the same, more particularly, to a photoreaction in which hydrogen can be efficiently and economically produced from water in the presence of a CdZnMS photocatalyst according to the present invention.
Hydrogen is generally used to produce ammonia and methanol and is applied to produce saturated compounds as an essential element. Also, it plays a pivotal role in hydrotreating processes, including hydrogen addition, desulfurization, denitrogenation, demetallization and especially the hydrogenation of carbon dioxide, which causes global warming. Furthermore, hydrogen is viewed as a pollution-free energy source and a substitute for existing fossil fuels.
There are many different kinds of conventional methods for producing hydrogen, which include extraction from fossil fuels, such as naphtha, modification of natural gas, a reaction of vapor with iron at a high temperature, a reaction of water with alkaline metal, and an electrolysis of water, etc.
However, these techniques are economically unfavorable because immense heat or electric energy is required and, particularly, in the modification of fossil fuels, a large quantity of carbon dioxide is generated as a by-product. For an electrolysis of water, problems including a short electrode lifetime and a by-product generated such as an oxygen should be solved. Thus, it has heretofore been economically unfavorable to solve these problems due to the huge cost for a hydrogen production facility.
Hydrogen gas can readily escape from the gravity of the earth because it is of low specific gravity, and most of it exists in water or inorganic forms. For these reasons, only a small amount of hydrogen exists in the atmosphere. It is also very difficult to purify hydrogen existing in an inorganic form. Even though hydrogen purification is practically possible, it is also economically unfavorable. Therefore, the development of technique for a high-purity hydrogen from water is very important for solving the urgent problem of exploiting substitute energy sources.
Recently, hydrogen producing techniques in which a photocatalyst is used to decompose water into hydrogen and oxygen have been developed. However, there is little published prior art relating to photocatalysts for producing hydrogen. Representative prior art is exemplified by Japanese Pat. Laid-Open Publication Nos. Sho 62-191045 and Sho 63-107815 and applications of present inventors as below.
Japanese Pat. Laid-Open Publication No. Sho 62-191045 shows that hydrogen is generated from a photolysis reaction of an aqueous Na2S solution in the presence of a rare-earth element compound. Also, the rare-earth element compound as a catalyst has an advantage of exhibiting an optical activity in the range of the visible light.
Japanese pat. Laid-Open Publication No. Sho 63-107815 describes a photolysis reaction in which a composite oxide of niobium and alkali earth metal is used as a photocatalyst to generate hydrogen from a methanol solution in water. Likely, this photocatalyst has an advantage of being optically active in a visible light.
However, the noted prior art is disadvantageous in that the amount of hydrogen generated is so small and the rate of hydrogen production is only 10 mL/0.5 g hr.
There are also Korean Pat. Appl""n. No.95-7721, No.95-30416 and No.96-44214, which are able to solve the above problems.
Korean Pat. Appl""n No. 95-7721 suggests a photocatalyst represented by the following general formula I:
Cs(a)/K4Nb6O17xe2x80x83xe2x80x83I
In the presence of the photocatalyst of formula I, this technique has little affect on the environment and can generate hydrogen at room temperature. However, the oxygen-containing organic compounds acting as a hydrogen-generating promoter to produce hydrogen make an interruption to reuse the reactants.
Korean Pat. Appl""n No.95-30416 suggests a photocatalyst represented by the following formula II:
Cs(a)M(c)/S(b)xe2x80x83xe2x80x83II
This technique also has little affect on the environment and can generate hydrogen without an oxygen-containing organic compound acting as a hydrogen-generating promoter at room temperature but has some problems with the lifetime and stability of said photocatalyst of formula II. For example, when alkali metal, such as cesium (Cs), is impregnated in a photo-carrier, the amount of hydrogen generated is outstandingly increased but the catalyst stability is decreased.
Korean Pat. Appl""n No. 96-44214 describes a photocatalyst represented by the following formula III:
Pt(a)/Zn[M(b)]Sxe2x80x83xe2x80x83III
This technique likewise has little affect on the environment. Although depending on electron donors and reducing agents, the photocatalyst of formula III is superior in simplicity of preparation, stability, and lifetime, as well as optical activity in the range of visible light, compared with previously-noted prior arts. But the amount of produced hydrogen is still economically unfavorable.
Korean Pat. Application No. 98-37179 suggests a photocatalyst represented by the following formula IV:
Pt(a)/Zn[M(b)]Sxe2x80x83xe2x80x83IV
This technique also has little affect on the environment and the said photocatalyst of formula IV has an optical activity in some degree in the range of visible light. The preparation of the said photocatalyst is much simpler and by-products are much less produced. However, the amount of generated hydrogen is still not enough economically.
To solve the above mentioned problems, Korean Pat. Application 98-37180 by present inventors suggests a photocatalyst represented by the following formula V:
m(A)/Cd[M(B)]Sxe2x80x83xe2x80x83V
The said photocatalyst of formula V shows an optical activity in the range of visible light adjusted by light filter as well as in the sunlight. The amount of generated hydrogen is much larger and the lifetime of the said photocatalyst is semi-infinitive. By introducing various doping metals and promoters and other new methods, said prior art overcomes the restricted activity in the light sources and suggests more simple method of preparation. Likewise, the lifetime of photocatalyst is also longer and the amount of generated hydrogen from water is remarkably larger than that of prior art. However, this technique shows limited hydrogen activity only to one reducing agent.
To solve the above mentioned problems economically, Korean Pat. Application 99-22954 by present inventors suggests a photocatalyst represented by the following formula VI:
xe2x80x83m(a)/Cd[M(b)]Sxe2x80x83xe2x80x83VI
In this prior art, the technique relates to novel CdS photocatalyst (photocatalyst system), preparation thereof and construction of new reduction system with a sulfite to generate hydrogen economically. However, the rate of producing hydrogen is still not satisfied in the economic point of view.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a novel photocatalyst for producing hydrogen with an optical activity in both visible and uv lights.
It is another object of the present invention to provide a photocatalyst with high activity in a reductant and sunlight, with a high yield of hydrogen, and with an infinite lifetime.
It is further object of the present invention to provide a preparation method for photocatalyst with a high degree of photocatalytic activity.
The photocatalyst of the present invention is characterized by the following general formula VII:
m(a)/CdxZnyMzSxe2x80x83xe2x80x83VII
wherein xe2x80x98mxe2x80x99 represents a doped metal element as an electron acceptor selected from the group consisting of Ni, Pt, Ru or the oxidized compound of these metals; xe2x80x98axe2x80x99 represents a percentage by weight of m, ranging from 0.10 to 5.00; xe2x80x98Mxe2x80x99 is at least one catalyst element selected from the group consisting of Mo, V, Al, Cs, Mn, Fe, Pd, Pt, P, Cu, Ag, Ir, Sb, Pb, Ga and Re. xe2x80x98zxe2x80x99 represents an atom % of M/(Cd+Zn+M), ranging from 0.05 to 20.00 and xe2x80x98xxe2x80x99 and xe2x80x98yxe2x80x99 represent an atom % of Cd/(Cd+Zn+M) and an atom % of Zn/(Cd+Zn+M), ranging from 10.00 to 90.00, respectively.
The preparation of photocatalyst in the present invention is characterized by the doping procedure as the following steps of: dissolving Cd-containing, Zn-containing and M-containing compounds in water in such an amount that the atom % of M ranging from 0.05 to 20.00 and the atom % of Cd/(Cd+Zn+M) and an atom % of Zn/(Cd+Zn+M), ranging from 10.00 to 90.00, respectively; adding H2S or Na2S as a reactant in the solution with stirring to precipitate CdZnMS; washing the precipitate with water and vacuum drying the precipitate in a nitrogen atmosphere; doping a liquid m-containing compound to this precipitate in such amount that the % by weight of m ranging from 0.10 to 5.00.
Likewise prior art of present inventors, hydrogen is produced by a method in which visible light adjusted by a light filter, sun light or uv light is irradiated onto a suspension of the said photocatalyst in water to which Na2S as an electron donor and NaH2PO2 or NaH2PO2 as a reductant have been added.
In detail, the present invention will be described as below.
Acting as an electron acceptor, the doping metal, m, in the formula VII is an element selected from the group consisting of Ni, Pt, Ru or an oxide thereof, and is used preferably at such an amount that the % by weight of m ranges approximately from 0.10 to 5.00. For example, if the amount of m ingredient is below 0.10% by weight, the amount of hydrogen generated is decreased and the stability of the said photocatalyst is also decreased. On the other hand, if the amount of m ingredient is over 5.00% by weight, the amount of hydrogen generated is decreased and the production cost is not economically favorable.
In the photocatalyst of the present invention, xe2x80x98Mxe2x80x99 is selected from the group consisting of Mo, V, Al, Cs, Mn, Fe, Pd, Pt, P, Cu, Ag, Ir, Sb, Pb, Ga, Re and xe2x80x98zxe2x80x99 represents an atom % of M/(Cd+Zn+M), ranging from 0.05 to 20.00. If the value of B is less than the lower limit, the activity of photocatalyst will be lost. On the other hand, if the value of B is over the upper limit, the amount of generated hydrogen will be decreased.
As to the molar ratio of Cd to S and Zn to S, it is preferred that the molar ratio of Cd to S ranges from 1:0.05 to 1.4 and that of Zn to S ranges from 1:0.05 to 1:1.4, more preferably, from 1:0.3 to 1:0.7, respectively. Within said molar ratio ranges, the efficiency of photocatalyst of the present invention is increased.
In the preparation of said photocatalyst, if xe2x80x98mxe2x80x99 is platinum (Pt) as a doping element, it is preferable for Pt to be illuminated with uv in a nitrogen atmosphere and doped on the CdZnMS by sintering. More preferably, hydrogen hexachloroplatinate(IV) (H2PtCl6) is added to the CdZnMS precipitate and irradiated with uv light in a nitrogen atmosphere to impregnate the carrier at such an amount that the value of m(Pt) ranges from 0.10 to 5.00. The precipitate thus obtained is washed with water until the wash water pH reaches 7, vacuum-dried at 105 to 130xc2x0 C. for 1.5 to 3.0 hours, oxidation-sintered at 300 to 400xc2x0 C. for 1.0 to 6.0 hours and then reduction-sintered at 300 to 400xc2x0 C. for 1.0 to 6.0 hours.
In case of other doping elements, the preferable preparation example of the photocatalyst comprises the steps of: adding an m-containing compound other than platinum to the CdZnMS precipitate obtained to reach the value of m ranging from 0.10 to 5.00; adding 6 or 7 drops of conc. hydrochloric acid with stirring; applying ultra sonication to the obtained slurry for 1.0 to 5.0 minutes; drying at 110 to 130xc2x0 C. for 1.5 to 3.0 hours in vacuo; oxidation-sintering at 300 to 400xc2x0 C. for 1.0 to 6.0 hours and then reduction-sintering at 300 to 400xc2x0 C. for 1.0 to 6.0 hours, to yield the said photocatalyst.
In the preparation of photocatalyst doped with platinum, the reason why it is dried and sintered at oxidation/reduction state after the pH reaches 7 is to keep electron acceptor, Pt, in pure state. As well known, when Pt in H2PtCl6 is irradiated with uv, Pt activates the surface of CdZnMS and makes a bond with separated S to form PtS and therefore a Wurzite structure is obtained by sintering under oxidation and reduction states at a temperature of from 300 to 400xc2x0 C. In case of sintering said product at a temperature of 300 to 400xc2x0 C. for 1.0 to 6.0 hours, Pt as an electron acceptor can be transferred to pure state of Pt(0). More preferably, it should be sintered at a temperature of from 320 to 390xc2x0 C. Beyond this temperature range, the lifetime and optical activity of said photocatalyst is decreased.
Examples of the Cd-containing compounds include CdCl2, CdBr2,,CdI2, Cd(CH3CO2)2 xH2O, CdSO4 xH2O, and Cd(NO3)24H2O and examples of the Zn-containing compounds include ZnCl2, ZnBr2, ZnI2, Zn(CH3CO2)2 xH2O, ZnSO4 xH2O and Zn(NO3)2xH2O and examples of the M-containing compounds include MoCl5, VCl3, VOSO4, VOCl3, Al(NO3)3, AlCl3, TiCl4, Cs2CO3, Ti[OCH(CH3)2]4, K2Cr2O7, Cr(CH3CO2)3,Cr(HCO2)3, Cr(NO3)3, H3PO2, NaH2PO2, SbCl3, MnCl3, MnF3, KMnO4, Pb(NO3)2, Pb(CH3CO2)4, RuCl3, FeCl3, IrCl3, Pd(NO3)2, H2PtCl6, Cu(NO3)23H2O, AgNO3, Ga(NO3)3, SnCl2, ReCl3 etc.
And then also examples of the m-containing compounds include H2PtCl6, RuCl3, NiSO4, Ni(NO3)2, Ni(CH3CO2)2, NiCl2, NiBr2, NiI2 etc.
In Korean Pat. Appl""n No. 96-44214, prior art of present inventors, etching with acid is required after the primary sintering, but in this present invention, only the step of drying the precipitate in vacuo in a nitrogen atmosphere is needed, so the steps for the primary sintering and etching with acid are not needed in this preparation.
However, according to the present invention, hydrogen is produced by dissolving from 0.15 to 1.00 mol of Na2S as an electron donor and from 0.15 to 1.00 mol of SO32xe2x88x92 instead of H2PO2xe2x88x92 as a reductant in primary and/or secondary distilled water or in the previously treated water, and adding the photocatalyst of the present invention thereto. Then, the thus-obtained suspension is irradiated with visible light adjusted by a light filter or uv light with stirring at a temperature of from 5 to 85xc2x0 C. at 0.1 atm. up to 5 atm. to yield hydrogen in a high degree of efficiency.
In addition, it is an important step to keep the concentration range of electron donor and reductant within the noted limits. If it is below the lower limit, the amount of hydrogen generated is decreased; if it is excess, the amount of hydrogen generated can not be increased further and the optimal reaction condition is at a temperature of from 10 to 60xc2x0 C. in from a vacuum to 2 atm.
The photocatalyst of the present invention has a semi-infinite lifetime if the electron donor and reductant are added repeatedly to the reaction.